The present invention relates to magnetic bearing devices of the control type, for example, for use in turbo-molecular pumps.
Magnetic bearing devices of the type mentioned comprise a rotor contactlessly supported by a plurality of control magnetic bearings and rotatable by an electric motor to be driven by an inverter. Electric power is supplied from an external power source, i.e., a commercial AC power source (200 V), to a power source unit for supplying electric power (direct current), for example, to the inverter, a magnetic bearing drive circuit and a main control unit for controlling these components. The power source unit rectifies and smooths the alternating current from the commercial power source and supplies the resulting current to the inverter, etc.
However, the conventional magnetic bearing device has the problem that when the input voltage from the power source unit to the inverter involves fluctuations due to voltage variations of the external power source, the motor current (the current actually flowing through the electric motor) also fluctuates correspondingly, consequently varying the rotational speed of the motor and failing to maintain the gas discharge performance of the pump at a constant level.
In the main control unit of the conventional magnetic bearing device, for example, in a PI control calculation unit thereof, the value of current to be supplied to the motor is calculated from the difference between a target value of rotational speed of the motor and the detected value of rotational speed thereof, and the calculated current value is output to the inverter as a command current value. Accordingly, even when the command current value to be output to the inverter is constant, the motor current varies if the input voltage from the power source unit to the inverter fluctuates.
The motor current variation can be eliminated by adding a constant-voltage circuit or like hardware to the main control unit, but the device then becomes large-sized and more costly to manufacture.